The present invention relates to a hydraulic (hydrostatic) vehicle drive system, and more particularly, to such a system for use in driving a pair (or pairs) of driven wheels from a single fluid source.
Many vehicles of the "off-highway" type are propelled by a hydraulic (hydrostatic) drive system wherein the drive (propel) wheels of the vehicle are directly driven by fluid pressure actuated hydraulic motors. Typically, such propel motors are either of the axial piston type, or of the radial piston (cam lobe) type, or of the gerotor type, although those skilled in the art will understand that the particular type of propel motor is not a significant feature of the present invention.
On some vehicles of the hydrostatic propel type, each wheel motor receives pressurized fluid from its own respective pump. On such a vehicle, an example of which would be a "skid steer loader", steering may be accomplished rather easily by reversing the direction of operation of the pump and motor on the side of the vehicle about which it is desired to effect a pivoting, steering movement.
However, on many vehicles of the hydrostatic propel type, there is only a single pump for the propel system, and the wheel motors form a parallel circuit, fed by the single pump. In a one pump, two (or four) motor propel system, performing very tight turns (small turning radius) is not as easy as in a vehicle such as a skid steer loader. An example of the type of vehicle to which the present invention relates is a large combine having a pair of front hydrostatic drive motors and a pair of rear hydrostatic drive motors. The front motors drive wheels which are not typically steerable, but can be, whereas the rear motors provide drive torque to wheels which are usually mounted on a steerable axle. The steerable axle is typically steered by means of a steering cylinder, with flow to and from the cylinder being controlled by a full fluid linked steering control unit (SCU), as is well known to those skilled in the art.
For any particular vehicle and drive system, there is a particular "kinematic" steering geometry which is possible, i.e., the smallest possible turning radius in which all of the wheels remain in rolling engagement with the ground, with no slipping or scrubbing at any wheel. On many vehicles, achieving kinematic steering provides an acceptable turning radius, but there are certain vehicles, such as certain large combines for which the vehicle manufacturers and operators find it desirable to be able to steer the vehicle (having one pump, and two driven wheels) in a turning radius which is even smaller than the kinematic radius. As is understood by those skilled in the art, the only way to achieve a turning radius less than kinematic is for one of the steerable wheels to scrub, i.e., to rotate slower than the vehicle is moving, thus resisting the vehicle forward motion on that side.
Unfortunately, it has been determined that, in order to achieve the desirable smaller turning radius, it is necessary to restrict the flow through the motor, on the downstream side of the motor, and providing such a restriction causes a back-pressure on the upstream side of the motor. The result of this back-pressure can include increasing the pressure at the inlet of the other motor (i.e., in the parallel combination of motors), and reducing the life of the various hydraulic components in the system. Another problem with simply restricting the oil flow upstream or downstream of the motor is that adequate reverse torque of the motor is not achieved. If the restriction is imposed upstream of the motor, no reverse torque is achieved. A downstream restriction does achieve reverse torque, but such reverse torque is limited by the inlet pressure or the pressure at the other propel motors, which can increase system pressure as noted previously.
As is well known to those skilled in the art, there are various other operating problems which arise in hydrostatic vehicle drive systems. On certain vehicle applications, such as turf equipment, a wheel may lose traction and "spin-out", causing damage to the lawn area on which the vehicle is traveling. On certain other vehicle types on which hydrostatic braking is used, going down hill can cause a pressure reversal on the pump and motors, and as the weight of the vehicle shifts forward, a loss of traction at one or both of the rear wheels can result in "scrubbing" of those wheels.